1. Field of the Invention
This invention relates to a sheet-feed press for printing single side/both sides, and more particularly to a structure of an intermediate cylinder in a sheet transfer system of such a sheet-feed press.
2. Description of the Prior Art
In a sheet-feed press for printing single side/both sides, a plurality of printing units including each plate cylinder, blanket cylinder, impression cylinder and ink arrangement are interconnected by a sheet transfer system referred to as a "double-size intermediate drum system". In other words, the upstream side printing unit and the downstream side printing unit are connected by the sheet transfer system consisting of a sheet transfer cylinder having an equal diameter to that of the plate cylinder of the printing unit, an intermediate cylinder having a diameter twice that of the plate cylinder and a reversing cylinder having a diameter equal to that of the plate cylinder, and the transfer of the sheet and reversing of the sheet necessary at the time of double-side printing are effected.
Accordingly, two sets of front edge grippers for gripping the front edge of the sheet and rear edge grippers for supporting the rear edge of the sheet only during double-side printing, that together form pairs, are provided to the intermediate cylinder in such a manner at to oppose one another with a 180.degree. phase difference between them with respect to a main shaft of the intermediate cylinder as the center. These gripper devices are generally referred to as "A grippers" and "B grippers", respectively.
During one-side printing, the sheet transfer cylinder receives the front edge of the sheet from the upstream side printing unit and delivers its front edge to the front edge grippers of the intermediate cylinder. The front edge grippers of the intermediate cylinder deliver the front edge directly to the grippers of the reversing cylinder at the contact point with the reversing cylinder that follows. Subsequently, the grippers of the reversing cylinder send to the front edge of the sheet to the downstream side unit. As a result, printing is made continuously on one side of the sheet. Incidentally, the rear edge grippers of the intermediate cylinder do not at all play any positive role during this one-side printing, and is substantially at rest.
During double-side printing, on the other hand, the sheet transfer cylinder receives the front edge of the sheet from the upstream side printing unit and lets the front edge grippers of the intermediate cylinder grip the front edge, in the same way as in the case of one-side printing. Subsequently, the rear edge grippers, too, operate, support the rear edge of the sheet and applies expansion and stretch to the rear edge so that the sheet can be wound tightly around the outer periphery of the intermediate cylinder. This is because the rear edge of the sheet becomes the reference for delivery of the sheet to the reversing cylinder in the case of double-side printing. In other words, the front edge grippers pass by the contact point with the reversing cylinder as such while gripping the sheet and when the rear edge grippers arrive at the contact point, the front edge grippers deliver the rear edge of the sheet to the reversing grippers of the reversing cylinder. Then, the reversing cylinder turns over the sheet with its rotation, and delivers the sheet to the downstream side printing unit, so that printing is made to the surface of the sheet opposite to the one printed by the upstream side printing unit.
Since the lengths of the sheets to be transferred are diversified, the position of the rear edge of the intermediate cylinder must be changed with respect to the front edge grippers fixed at predetermined positions, in accordance with the sheet size when double-side printing is carried out.
U.S. Pat. No. 4,204,471 discloses the structure of an intermediate cylinder wherein a rear edge gripper holder is movable and adjustable with respect to a front edge gripper holder in order to adjust the size of the sheet of the intermediate cylinder in double-side printing. In this prior art, the front edge gripper holder 50 and the rear edge gripper holer 52 fit to one another in a comb-shape and define a perfectly circular intermediate cylinder when the outer periphery is viewed as a whole. Here, the term "comb-shape" means the arrangement wherein the outer surface of the front edge gripper holder 50 is notched in a comb-shape and comb-shaped blades 56 of the rear edge gripper holder 52 fit into these notches 54.
However, if the comb-shape is required for both the front edge gripper holder and the rear edge gripper holder, many man-hours are necessary for the production, assembly and adjustment of these components and moreover, if the end portion of the sheet hangs down into the comb-shape notches, a trouble would occur during the sheet delivery.